Mechanistic and computational studies of the atom transfer radical addition of CCl4 to styrene catalyzed by copper homoscorpionate complexes

Article abstract of DOI:10.1021/ic102279w

Experimental as well as theoretical studies have been carried out with the aim of elucidating the mechanism of the atom transfer radical addition (ATRA) of styrene and carbon tetrachloride wwith a Tp^xCu(NCMe) complex as the catalyst precursor (Tp^x = hydrotrispyrazolyl-borate ligand). The studies shown herein demonstrate the effect of different variables in the kinetic behavior. A mechanistic proposal consistent with theoretical and experimental data is presented.

Full text of DOI:10.1021/ic102279w

ARTICLE
pubs.acs.org/IC
Mechanistic and Computational Studies of the Atom Transfer
Radical Addition of CCl to Styrene Catalyzed by Copper
4
Homoscorpionate Complexes
†
‡
§
||,^
,†
ꢀ
Jos ꢀe María Mu ~n oz-Molina, W. M. C. Sameera, Eleuterio Alvarez, Feliu Maseras, Tom ꢀa s R. Belderrain,*
,†
and Pedro J. P ꢀe rez*
†
Laboratorio de Cat ꢀa lisis Homog ꢀe nea, Departamento de Química y Ciencia de los Materiales, Unidad Asociada al CSIC,
Centro de Investigaci ꢀo n en Química Sostenible (CIQSO), Universidad de Huelva, Campus de El Carmen, 21007 Huelva, Spain
‡
Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR,
United Kingdom
§
CSIC-Universidad de Sevilla, Avenida de Am ꢀe rico Vespucio, 49, 41092 Sevilla, Spain
Institute of Chemical Research of Catalonia (ICIQ), 43007 Tarragona, Catalonia, Spain
^
Departament de Quimica, Universitat Autonoma de Barcelona, 08193 Bellaterra, Catalonia, Spain
S Supporting Information
b
ABSTRACT: Experimental as well as theoretical studies have been
carried out with the aim of elucidating the mechanism of the atom
transfer radical addition (ATRA) of styrene and carbon tetrachloride
x
x
with a Tp Cu(NCMe) complex as the catalyst precursor (Tp =
hydrotrispyrazolyl-borate ligand). The studies shown herein de-
monstrate the effect of different variables in the kinetic behavior. A
mechanistic proposal consistent with theoretical and experimental
data is presented.
’
INTRODUCTION
center undergoes a reversible oxidation-reduction process,
changing its oxidation state on one unit.
Several metals have been described to catalyze this transfor-
The addition of a polyhalogenated saturated hydrocarbon to
an olefin, namely, the Kharasch reaction, constitutes an efficient
way to generate carbon-carbon bonds (Scheme 1a). This reac-
tion may occur in the presence either of a free-radical precursor as
the halogen transfer agent or of a transition metal complex as the
catalyst. In the latter case, the so-called atom transfer radical
1
mation, which has been described in both the inter- and intra-
2
3
molecular versions. Early work focused in ruthenium- or nickel-
4
based catalysts showed very high activities for the intermolecular
case, whereas the use of copper was restricted to the intramolec-
2
e
2
ular atom transfer radical cyclization (ATRC) reaction. How-
ever, in the past decade the use of copper complexes as catalysts
in the intermolecular reactions has emerged with excellent
addition (ATRA) reaction, the transformation has been ex-
plained in terms of three steps, two of which are metal-mediated.
The first step proceeds via a metal-induced homolytic cleavage of
the carbon-halogen bond, affording a metal-halide complex
and a carbon-centered radical (activation step). Subsequently,
the latter species interacts with the olefin to form a second radical
species which induces the abstraction of halogen from the above
metal-halide complex (deactivation step). Therefore, the metal
2a
results. Thus, Pintauer and co-workers have reported the effi-
cient ATRA of polyhalogenated alkanes to olefins such as 1-hexene
Received: November 13, 2010
Published: February 14, 2011
r 2011 American Chemical Society
2458
dx.doi.org/10.1021/ic102279w
|Inorg. Chem. 2011, 50, 2458–2467

Inorganic Chemistry
ARTICLE
Scheme 1. (a) Kharasch Reaction and (b-d) Commonly
Proposed Mechanism for the Metal-Mediated Atom Transfer
Radical Addition (ATRA) Reaction
tBu,Me
Figure 1. Molecular structure of Tp
Cu(NCMe) (1) (30% dis-
Scheme 2. Previous Mechanistic Proposal for ATRA
Reactions Catalyzed by Tp Cu Complexes
placement ellipsoids, hydrogen atoms have been omitted except for that
bound to the boron atom).
x
tBu,Me
Scheme 3. Synthesis of the Catalyst Precursor Tp
NCMe) (1)
Cu-
(
or 1-octene (with TONs as high as 160 000 with 5 ppm of catalyst
tBu,Me
5
reaction of the thallium salt of the corresponding Tp
ligand
loading). Our group has recently described that copper-homo-
and copper chloride in acetonitrile as the solvent (Scheme 3).
Spectroscopic data were in agreement with the proposed for-
mula. Since the catalytic system studied in this contribution is
markedly influenced by the copper species that exists in the
reaction mixture, we have carried out X-ray diffraction studies to
unambiguously assign the nature of the catalyst precursor. As
shown in Figure 1, complex 1 displays a structure that corre-
sponds to a distorted tetrahedron around the metal center with
the N-C-Me group slightly displaced from the ideal B-Cu
axis. This feature, as well as distances and angles, are quite similar
scorpionate complexes catalyze the atom transfer radical addition of
6
7
polyhalogenated alkanes to terminal olefins or to R,ω-dienes,
affording in the latter case five-membered carbocycles by a con-
current intramolecular radical process. Although the overall mecha-
nism of this transformation has been accepted as that shown in
Scheme 1, the intimate nature of the different steps as well as the
effect of the different variables has not yet been fully described. In
6b
our first contribution in this area, we proposed a plausible mecha-
nism for these ATRA reactions (Scheme 2) catalyzed by complexes
x
x
Tp Cu (Tp = hydrotrispyrazolylborate ligand) on the basis of
kinetic data obtained. Since the knowledge of the mechanism would
afford the design of more active catalysts, we have decided to carry
out a complete mechanistic study on an ATRA model reaction,
x
8
to other already reported Tp Cu(NCMe) complexes and
deserve no additional comments.
Once the nature of the precatalyst was established, we studied
its behavior in solution. Complex 1 was dissolved in C D , and
including experimental as well as theoretical calculations, using
6 6
1
tBu,Me
the solution was investigated by H NMR. In addition to that of
Tp
Cu(NCMe) as the catalyst (1, Scheme 3). A complete
tBu,Me
9
1
(
, a second set of resonances corresponding to [Tp
2) was observed, growing with time although at a very low rate.
Figure 2 shows the kinetics of this process, in which acetonitrile
dissociates from the copper center with a k
= 1.43 ꢀ
Cu]2
mechanistic picture is now proposed that explains not only the route
leading to the formation of the addition product but also the impor-
tant role of all the reagents in the control of the catalyst precursor
and therefore in the well-known persistent radical effect equili-
obs,init
-
4
-1 -1
2b
1
0
M
s
. In order to ascertain if an equilibrium between
brium, which is crucial for this transformation.
x
these acetonitrile adducts and the dimeric [Tp Cu] could exist
2
in solution, 1 equiv of MeCN (referred to Cu) was added to a
’
RESULTS AND DISCUSSION
tBu,Me
solution of previously prepared [Tp
Cu] (2). After 100 h at
2
Catalyst Precursor: Structure and Behavior in Solution.
The complex Tp
room temperature, the relative concentrations of 1 and 2 were
measured by integrating the peaks corresponding to the Bu,
tBu,Me
t
Cu(NCMe) (1) was prepared by direct
2
459
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Inorganic Chemistry
ARTICLE
Scheme 4. Solution Equilibria Involving the Catalyst Pre-
tBu,Me
cursor Tp
Cu(NCMe) (1) and Styrene
Scheme 5. Equilibria Affecting the Formation of Cu(II)
Species
tBu,Me
Figure 2. Kinetics of dissociation of acetonitrile from Tp
Cu(NCMe) in C D .
6 6
-
-
2
leading to Keq = 3.2 ꢀ 10 (eq 1).
-
CH3CN
TptBu,MeCuðNCMeÞ R
1=2½TptBu,MeCuꢁ2
F
(in the absence of styrene) did not provide any information: after
þCH3CN
tBu,Me
2
4 h most of the initial Tp
Cu(NCMe) remained unreacted.
1
2
ð1Þ
But the use of a 1:200 ratio, for example, a large excess of CCl4
-2
simulating the catalytic transformation, led to the formation of
Kep ¼ 3:2 ꢀ 10
tBu,Me
9
the Cu(II) complex Tp
metric amount of hexachloroethane, the product resulting from
CuCl (5) as well as the stoichio-
It seems reasonable to assume that these transformations occur
through the intermediacy of the coordinatively unsaturated
Cu (Scheme 4). In fact, we have recently de-
monstrated that the olefin exchange processes in Tp Cu(olefin)
complexes take place by means of a dissociative mechanism,
which involves such Tp Cu intermediates. The addition of a
0-fold excess of styrene to a solution of complex 1 did not
induce the formation of any olefin adduct in detectable amounts
in the NMR detection limit. A similar experiment using the
dinuclear complex 2 and styrene showed the formation of
Cu(styrene) (3) to some extent. The existing equilibria
in solution when complex 1 is dissolved in a styrene solution
seem to be those shown in Scheme 4. Slow dissociation of the
MeCN takes place to generate Tp
brium with the dinuclear and the styrene complexes 2 and 3,
respectively. In spite of such equilibria, it should be pointed out
that this system is quite simple compared with other already
described systems for this transformation, in which a copper(I)
source and the corresponding ligand are mixed in situ. A very
•
the homocoupling of two CCl radicals (Scheme 5) after 4 h.
3
tBu,Me
species Tp
Interestingly, a second experiment carried out adding 20 equiv of
acetonitrile with respect to the catalyst precursor induced a
significant decrease in the reaction rate of the formation of
complex 5. Therefore, all these pieces of information seem to
favor the existence of an equilibrium between the unsaturated
x
x
10
1
tBu,Me
tBu,Me
species Tp
Cu (U) and the CCl
adduct Tp
CuClCCl
4
3
previously to the formation of the copper(II) chloride complex 5.
•
The latter step supposes the formation of the CCl radical that
3
tBu,Me
Tp
undegoes the homocoupling step. It is interesting to note that
when styrene is added, that is, under catalytic conditions, hexa-
chloroethane is not observed in the NMR detection limit. The
effect of the added acetonitrile is explained by the control exerted
in the dissociation equilibrium from the catalyst precursor 1.
A well-known feature of atom transfer radical reactions is the
persistent radical effect. The equilibrium between Cu(I) and Cu(II)
species is crucial to control the amount of radicals delivered into the
reaction medium and therefore to avoid side, undesired reactions.
When the concentration of radicals is maintained at low values,
radical-to-radical coupling is minimized and therefore the selectivity
tBu,Me
Cu that is also in equili-
11
elegant work from Matyjaszewki’s group have recently shown
that a number of different species with distinct metal-to-ligand
ratios are present in solution, therefore affecting the catalytic
toward the desired products (addition of CCl
to the CdC bond) is
4
x
behavior. In our system, there Tp Cu unit remains as a core along
maximized. In our case, the concentration of the Cu(II) species 5
•
the reaction time, only one coordination site being available for
the catalytic process to occur.
Reaction of the Copper Catalyst Precursor 1 with CCl4.
and subsequently the concentration of CCl are controlled by the
3
concentration of the unsaturated intermediate U. As explained
above, loss of acetonitrile from complex 1 is slow, and because of
its dissociative nature, it can be directed with the addition of free
MeCN. Overall, in our system the persistent radical effect equilib-
rium could be written as in eq 3.
We have chosen the ATRA reaction of styrene and CCl as the
4
model for this study (eq 2).
This is the probe reaction commonly employed to test the
validity of a given complex as catalyst for this transformation.
However, synthetic application using this polyhalomethane
reagent in radical addition reactions has been also described.
Once the pre-equilibria existing in solutions of complex 1 and
Due to the equilibria shown in Schemes 4 and 5, the concentra-
tion of 5 in solution is very low in comparison with that of 1,
particularly when free MeCN is added for such purpose.
12
styrene (Scheme 4) are established, the reaction with CCl was
In our previous proposal (Scheme 2), we assumed that the
4
tBu,Me
studied. The stoichiometric [1]:[CCl ] in benzene-d at 30 ꢀC
Cu(II) complex Tp
CuCl (5) could be trapped by either the
4
6
2
460
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Inorganic Chemistry
ARTICLE
olefin or an added donor, acetonitrile in this case, to give
tBu,Me
Tp
CuCl(NCMe), and that this would be the crucial species
to initiate the deactivation step. To test this, we have now studied
the solution behavior of complex 5 in the presence and absence
of acetonitrile, using toluene as the solvent. Figure 3 shows the
UV-vis spectrum of this complex in toluene that did not vary
when excess of acetonitrile was added. Therefore, our previous
tBu,Me
assumption that the pentacoordinated complex Tp
NCMe) was formed is no longer valid.
Effect of the Variables in the Catalytic Reaction. We have
CuCl-
(
carried out kinetic experiments to study the effect that a variation
in [catalyst], [styrene], and [CCl ] induce in the catalytic reac-
4
tion. In the first case, as shown in Figure 4, a linear dependence
tBu,Me
of kobs vs [1] was observed ([1] corresponds to the total
Figure 3. Electronic spectrum of Tp
CuCl (3) in toluene. Iden-
0
0
concentration of copper species in solution, [Cu] ). A second
tical wavelengths were observed in the presence as well as in the absence
of CH CN [398 nm (1532) and 875 nm (68)] [λ nm (ε M cm )].
tot
-
1
-1
set of experiments in which styrene concentration was varied
3
(
Figure 5) indicated an inhibitory effect of the olefin, as inferred
from the decrease of kobs (rate constant for product formation)
when increasing [styrene]. Finally, the effect of [CCl ] led to a
4
saturation curve (Figure 6). We have also analyzed the depen-
dence of the rate on the reaction temperature to estimate the
activation parameters for the ATRA reaction of styrene and CCl4
catalyzed by complex 1. Figure 7 contains the data collected as an
Eyring plot. Good linear correlation analysis, when fitted to the
standard Eyring equation, has provided the activation parameters
‡
‡
ΔH = 29.6(3) kcal/mol and ΔS = 19.3(2) cal/(mol K), for
‡
ΔG = 23.8(5) kcal/mol.
Therefore, any mechanistic proposal should explain the ex-
perimental observations, which are (i) linear dependence with
catalyst concentration, (ii) inhibitory effect of acetonitrile, (iii)
inhibitory effect of [styrene], (iv) saturation effect of [CCl ], and
4
‡
(v) ΔS > 0. The inhibitory effects of both acetonitrile and olefin
Figure 4. Dependence of kobs with the concentration of the catalyst
precursor 1, for the ATRA reaction of styrene and CCl4.
could be readily explained by the equilibria shown in Scheme 4,
the higher the amounts of MeCN and/or styrene in the
reaction mixture, and the lower the concentration of
tBu,Me
Tp
Cu (U) to react with CCl . The saturation effect when
4
increasing [CCl ] was already observed by Van Koten and co-
4
4
c
workers with nickel-based catalysts; an explanation is yet to
be made. The positive value of the activation entropy could be
in agreement with the dissociation of acetonitrile from 1 as the
rate determining step. In other cases, it has been proposed that
the activation step, that leading to the fomation of 5 and the
•
CCl radical, is the rate determining step, through the inter-
3
1
3
mediacy of an adduct-radical. But the formation of the such
adduct radical would involve an early transition state more
ordered than the reactants, in contrast with that positive
entropy parameter.
There is one major issue yet to be evaluated. It is well-known
that in these catalytic systems the bimolecular radical coupling
originates a secondary effect, the accumulation of Cu(II) species
that cannot return to the lower oxidation state. In this manner, the
catalyst deactivates with time and conversions are lowered. In
Figure 5. Dependence of kobs for product formation with the olefin
concentration for the ATRA reaction of styrene and CCl
Conditions: [cat] = 4.6 mM; solvent, benzene-d
4
catalyzed by 1.
6
.
Rate Law. On the basis of the collected data, Scheme 6 shows
the different reactions that should be taken into account to explain
this transformation. Styrene consumption would be given by
order to evaluate such effect, we have monitored by UV-vis the
tBu,Me
accumulation of Cu(II), in our case, the presence of Tp
-
CuCl (5), in solution during the catalytic reaction. The initial [1]
-3
was 1.4 ꢀ 10 M, and after 3 min at 20 ꢀC, [5] was estimated as
d½styꢁ
•
-
4
-4
-
¼ k2½ CCl3ꢁ½styꢁ
ð4Þ
2
.0 ꢀ 10 M. The values at 2.5 and 14 h were 2.3 ꢀ 10 M,
dt
indicating that the Cu(I)-Cu(II) equilibrium is reached in the
first stages of the catalytic reaction and that no accumulation takes
place (see Supporting Information). The explanation for this
behavior should invoke a highly favorable deactivation step in
which 5 and the corresponding radical would collapse easily.
•
The variation of the concentration of the CCl with time would be
3
•
d½ CCl ꢁ
3
•
•
-
¼ k1½4ꢁ - k-1½5ꢁ½ CCl3ꢁ - k2½ CCl3ꢁ½styꢁ
dt
2
461
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Inorganic Chemistry
ARTICLE
Scheme 6. Different Steps in the ATRA Reaction of Styrene
and CCl Using Complex 1 as the Catalyst Precursor
4
4
Figure 6. Dependence of kobs with the CCl concentration for the
ATRA reaction of styrene and CCl catalyzed by 1. Conditions: [cat] =
4
4.6 mM; solvent, benzene-d6.
This equation needs to be written as a function of the total Cu
concentration, which is the initial concentration of the catalyst
precursor 1. At this stage, we introduce a second assumption: to
account for the total concentration of copper species in solution,
Figure 7. Eyring plot for the ATRA reaction of styrene and CCl4
catalyzed by 1. Conditions [1]:[styrene]:[CCl
.5 mM; solvent, benzene-d
4
] = 1:300:1200; [cat] =
only the acetonitrile, styrene, and CCl adducts as well as the
4
1
6
.
tBu,Me
unsaturated intermediate Tp
Cu (U) will be considered. The
already mentioned steady-state condition for 5 and the experimental
data previously commented on showing that, in the presence of
acetonitrile or olefin, the dinuclear species 2 does not exist in
solution support this second approximation. We could employ the
experimental data collected for [5] as a fraction of initial [1] but will
stand on our initial assumption.
At this stage, and in order to simplify the kinetic calculations, we
propose a first approximation: since the concentration of 5 remains
constant along the reaction time, we have assumed steady state
•
conditions for it as well as for CCl . Therefore, we assume that
3
•
d½ CCl3ꢁ
-
¼ 0
dt
½Cuꢁtot ꢂ ½1ꢁ þ ½3ꢁ þ ½4ꢁ þ ½Uꢁ
•
•
k1½4ꢁ - k2½ CCl3ꢁ½styꢁ . - k-1½5ꢁ½ CCl3ꢁ
Again, from different equilibria we obtain [1], [3], and [4] as a
function of [U]:
•
From which the value of [ CCl ] is calculated as
3
½
Uꢁ½MeCNꢁ
½Uꢁ½MeCNꢁ
k1½4ꢁ
KL ¼
w ½1ꢁ ¼
•
½
CCl ꢁ ¼
½1ꢁ
K
3
L
k2½styꢁ
½
3ꢁ
And substituting above we end that
KO ¼
w ½3ꢁ ¼ KO½Uꢁ½styꢁ
½
Uꢁ½styꢁ
d½styꢁ
-
¼ k1½4ꢁ
dt
½
4ꢁ
KC ¼
w ½4ꢁ ¼ K ½Uꢁ½CCl ꢁ
C
4
From equilibria we can write
½Uꢁ½CCl ꢁ
4
½
4ꢁ
The value for [Cu]tot is therefore given by
KC ¼
w ½4ꢁ ¼ K ½Uꢁ½CCl ꢁ
C
4
½
Uꢁ½CCl4ꢁ
½
Uꢁ½MeCNꢁ
½
Cuꢁtot
¼
¼
þ ½Uꢁ þ KO½Uꢁ½styꢁ þ KC½Uꢁ½CCl4ꢁ
KL
And substituting above
ꢀ½
ꢁ
MeCNꢁ
d½styꢁ
½Uꢁ
þ 1 þ KO½styꢁ þ KC½CCl4ꢁ
-
¼ k1KC½Uꢁ½CCl4ꢁ
KL
dt
2
462
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Inorganic Chemistry
ARTICLE
and
Scheme 7. Mechanistic Proposal for the ATRA Reaction
Catalyzed by Tp Cu(NCMe)
x
½
Cuꢁtot
½
Uꢁ ¼
½
MeCNꢁ
KL
þ 1 þ KO½styꢁ þ KC½CCl4ꢁ
Substitution in the equation for styrene consumption provides
ð5Þ
d½styꢁ
dt
k1KC½CCl4ꢁ½Cuꢁtot
-
¼
½MeCNꢁ
þ 1 þ KO½styꢁ þ KC½CCl4ꢁ
K
L
Equation 5 shows the rate law obtained under the mechanistic
picture shown in Scheme 6, and with the discussed approximations,
their validity now depends on its use to explain the experimental
results. In this sense, this equation is in very good agreement with all
the experimental data available. Thus, the linear dependence with
catalyst concentration comes straight from such equation. Similarly,
the inhibition of the process in acetonitrile as the solvent as well as
the observation of lower reaction rates with added acetonitrile come
from an inverse dependence with [MeCN]. At large [MeCN], and
given the low value of K , [MeCN]/K is quite large, leading to no
L
L
conversion experiments. The same inhibitory effect could be applied
to the olefin, since an increase in [sty] would imply a decrease in
olefin consumption.
Although the above mechanistic proposal is in agreement
with all data collected, we decided to carry out theoretical calcul-
ations that provide additional information about the overall
mechanistic picture, with particular emphasis in two issues:
(a) the rate determining step, which we have located in the
dissociation of acetonitrile from complex 1, and (b) the deactiva-
tion step. In the latter case, an alternative explanation would
The saturation effect observed at high [CCl ] can now be
4
explained in a simple manner: at large values of [CCl ] we can
4
assume that
½
MeCNꢁ
KL
invoke the existence of a pentacoordinated intermediate of com-
þ 1 þ K ½styꢁ þ K ½CCl ꢁ ꢂ K ½CCl ꢁ
O
C
4
C
4
tBu,Me
position Tp
Cu(NCMe)Cl due to the trapping of 5 with
acetonitrile. Such pentacoordinated intermediate would collapse
and hence
with the carbon-centered radical PhCHCH CCl to afford the
2
3
d½styꢁ
dt
k1KC½CCl4ꢁ½Cuꢁtot
final product as well as complex 1 to restart the catalytic cycle.
Theoretical Studies. We first carried out a series of DFT and
DFT/MM calculations. We modeled the ATRA of CCl to C H
2 4
-
¼
¼
½MeCNꢁ
KL
þ 1 þ KO½styꢁ þ KC½CCl4ꢁ
4
tBu,Me
catalyzed by Tp
Cu(NCMe) in the presence of MeCN. The
resulting potential energy profiles are presented in Figures 8
and 9. The first profile, Figure 8, deals with the equilibria related
to ligand exchange at the copper center prior to the C-Cl bond
cleavage. In the presence of three potential ligands, as MeCN,
k12KC½CCl4ꢁ½Cuꢁ
K ½CCl ꢁ
tot
¼ k2½Cuꢁtot
C
4
At the saturation limit, kobs = k [Cu] , and k can be obtained
2
tot
1
from the flat region of the curve in Figure 6; an estimation of the
C
2
H
4
, and CCl
, there is an equilibrium between four Cu(I)
4
tBu,Me
-
4
-1
value of k as 2.4 ꢀ 10
s
has been obtained.
species: the three-coordinate complex Tp
four-coordinate species Tp
Cu(C
Cu (U) and the
1
tBu,Me
tBu,Me
In a separate experiment similar to that described by Matyjaszewski
Cu(NCMe) (1), Tp
-
14
tBu,Me
and co-workers, 1was reacted with CCl in the presence of TEMPO
4
H
4
), and Tp
Cu(CCl
4
) (4). The acetonitrile adduct
4
(
as excess radical trapping agent) to directly obtain the value of
1 produces the most stable species in this context and will be
-4 -1
k , which was found to be 2.0 ꢀ 10 s , in good agreement with
used as origin of energies for all compounds discussed. This
1
tBu,Me
the above. We believe that this cross-check experiment assesses the
validity of the kinetic study, from which the following mechanistic
proposal can be also proposed.
Tp
Cu(NCMe) species is followed in stability by the
1
complexes with C
respectively. The highest energy corresponds to the three-
coordinate complex Tp
H and CCl
, 9.8 and 14.1 kcal mol- above,
2 4 4
tBu,Me
-1
Mechanistic Proposal. On the basis of the previous and new
experimental data, Scheme 7 displays a mechanistic proposal for
complex 1 as the catalyst precursor. The first step would be the
dissociation of the acetonitrile molecule, the presence of added
acetonitrile being crucial to regulate the amount of the real
catalytic species, Tp
tion of radicals in the reaction mixture. The next step would
consist of the interaction of Tp
called activation step, that would provide an adduct radical prior
to the generation of Tp
Cu, 21.5 kcal mol above the
MeCN complex. All these complexes hold fully occupied metal
10
d orbitals (d ). Therefore, the four-coordinate complexes are
18-electron species, and all ligand exchange reactions proceed
tBu,Me
likely by a dissociative mechanism, going through Tp
Cu.
tBu,Me
Cu, and, subsequently, the concentra-
This is in agreement with previous experimental and theoretical
x 10
studies from these laboratories with Tp Cu(olefin) complexes.
tBu,Me
tBu,Me
-1
Cu with CCl , in the so-
The relatively high energy of Tp
the Tp
Cu, 21.5 kcal mol above
Cu(NCMe), is the first barrier that must be over-
come for the catalytic cycle to proceed. It is worth mentioning
4
tBu,Me
tBu,Me
•
CuCl (5) and CCl . At this stage, in
3
•
tBu,Me
the nonmetal mediated route, CCl and styrene would react to
that the Tp
Cu(CCl ) complex, the one that will open
3
4
give another radical that further reacts with 5 in the final,
deactivation step.
the reaction channel for the ATRA process, is less stable than the
MeCN and C H counterparts. This is in agreement with the
2
4
2
463
dx.doi.org/10.1021/ic102279w |Inorg. Chem. 2011, 50, 2458–2467

Inorganic Chemistry
ARTICLE
-
1
experimental observation that an increase in either acetonitrile or
olefin can slow down the overall process.
dissociation limit, which is about 7.0 kcal mol above the
tBu,Me
Tp
CuCl-CCl3 intermediate. At the beginning of the
The second computed profile, Figure 9, shows the reaction
C-Cl bond cleavage, the spin density on each atomic center is
tBu,Me
from the Tp
starting species may be seen as a Tp
because only one of the chlorine atoms of CCl is interacting with
Cu(CCl ) complex to the final products. This
zero but spin localization develops as the homolytic cleavage
4
tBu,Me
CuCl-CCl adduct,
advances toward the two separate doublet systems. The disso-
3
•
ciated CCl radical can attack the olefin with a low barrier of
4
3
-
1
-1
copper. The Cu-Cl distance is quite long, 2.609 Å. Homo-
5.7 kcal mol and an exothermicity of -17.8 kcal mol
(not shown in the profile). The resulting CH CH CCl radical
•
lytic cleavage of the C-Cl bond produces the species
2
2
3
tBu,Me
•
[
Tp
CuCl]...[ CCl ], a Cu(II) complex at the C-Cl bond
rejoins then the energy profile in Figure 9 to yield the product.
3
We have considered two possibilities: this new radical can either
tBu,Me
attack directly the Tp
coordinate Tp
CuCl intermediate or attack a penta-
tBu,Me
CuCl(NCMe) species that may be formed in
solution, as a bifurcation shown in the profile. The direct reaction
between the radical and the four-coordinate Cu(II) intermediate
is barrierless in terms of potential energy, while the reaction
through the five-coordinate intermediate goes through an addi-
-
1
tional barrier of 6.4 kcal mol . Therefore, the five-coordinate
species with chloride and acetonitrile simultaneously bonded to a
copper(II) center should not be formed, in good agreement with
the UV-vis studies. The reaction will therefore proceed through
tBu,Me
attack of the organic radical on the Tp
CuCl complex, in an
-
1
exothermic step by 30.1 kcal mol .The slightly endothermic
-
1
(
10.3 kcal mol ) separation of the product will then yield the
tBu,Me
Tp
Cu complex, which can reinitiate the catalytic cycle upon
carbon tetrachloride coordination, or it can coordinate again
acetonitrile or olefin and re-enter the equilibria described in
Figure 8.
The computed energy profiles confirm the validity of the
mechanistic proposal in Scheme 7 and add further details on
tBu,Me
the catalytic cycle. The three-coordinate Tp
Cu(I) inter-
Figure 8. Potential energy diagram for the equilibria connecting the
initial form of the catalyst with the reaction channel (energy values are in
kcal mol and spin densities at selected atoms are shown in italics).
mediate is the species with the highest energy in the whole
process, and its formation is the key for the reaction rate.
-
1
Figure 9. Potential energy diagram for the ATRA of CCl to C H by the TptBu,MeCu(I) complex. (Energy values are in kcal mol-1, and spin densities at
4
2 4
selected atoms are shown in italics.)
2
464
dx.doi.org/10.1021/ic102279w |Inorg. Chem. 2011, 50, 2458–2467

Inorganic Chemistry
ARTICLE
The presence in excess of coordinating agents such as aceto-
nitrile or olefin will reduce the concentration of this high
The solution was transferred into a pressure NMR tube and sealed with a
Teflon screw cap. The tube was removed from the glovebox and placed
in an oil bath. The temperature was controlled in the range 10-50 ꢀC
energy intermediate and can thus slow down the reaction. The
tBu,Me
1
Tp
Cu(II)Cl intermediate will necessarily have a low con-
((0.1 ꢀC). The conversions were monitored by H NMR spectroscopy at
the desired times. The Eyring plot for the reactions are given in Figure 7.
Activation parameters were calculated over the temperature range from the
least-squares fit of values of ln(k/T) to 1/T, according to the equation
centration, because it reacts without barrier with the coexisting
radical species in solution, thus confirming the validity of the rate
law presented above.
ꢀ
ꢁ
ꢀ
ꢁ
‡
‡
k
kB
ΔS
ΔH
’
CONCLUSIONS
ln
¼ ln
þ
þ
RT
T
h
R
A complete mechanistic proposal for the model Atom Trans-
tBu,Me
fer Radical Addition of CCl to styrene using Tp
Cu(NCMe)
Kinetics of the Reaction of CCl to Styrene at Different CCl
4 4
4
-3
Concentrations. Complex 1 (4.1 ꢀ 10 mmol from a stock solution),
as the catalyst precursor has been made. The importance of several
equilibria between Cu(I) species to control the amount of the real
catalytic species has been established. This seems crucial since that
concentration directly affects the concentration of radicals in
solution and, when in the appropriate range, allows the process
to be directed toward the desired product and, subsequently, avoids
the formation of other nondesired products derived from radical
coupling reactions.
styrene (300 equiv, 1.23 mmol), and the desired amount of CCl (10, 60,
4
300, 600, and 1200 equiv, respectively) were dissolved in the required
amount of C to reach a total volume of 900 μL. These five solutions
6 6
D
were transferred into pressure NMR tubes and sealed with Teflon screw
1
caps. The conversions were monitored by H NMR spectroscopy.
Kinetics of the Reaction of CCl to Styrene at Different
Olefin Concentrations. Complex 1 (4.1 ꢀ 10 mmol from a stock
4
-
3
solution), CCl (3000 equiv, 12.3 mmol), and the desired amount of
4
styrene (100, 200, 300, 600, and 1200 equiv with respect to catalyst)
were dissolved in the required amount of C D to reach a total volume of
’
EXPERIMENTAL SECTION
6
6
General Information. All preparations were carried out in a
900 μL. The solutions were transferred into five pressure NMR tubes
glovebox, placed into a thick walled pressure NMR tube, and sealed with
a Teflon screw cap under nitrogen. Starting materials and reagents were
purchased from Aldrich and were purified as follows: carbon tetrachloride
was distilled, and styrene was filtered on alumina columns prior to use.
and sealed with Teflon screw caps. The conversions were monitored by
H NMR spectroscopy.
1
Kinetics of the Reaction of CCl
Catalyst Concentrations. Complex 1 (33, 66, 100, 150, and 200 μL
from a 0.05 M stock solution), styrene (1.23 mmol), and CCl (4.92
mmol) were dissolved in the required amount of C to complete a
to Styrene at Different
4
15,16
tBu,Me
9
The homoscorpionate ligand
and the [Tp
Cu]2 (2) complex
4
were prepared according to literature methods. NMR experiments were
run in a Varian Mercury 400 MHz spectrometer. UV-vis spectra were
recorded in a 1 cm quartz cuvette under nitrogen using a Cary 50 UV/vis
spectrophotometer (Varian Scientific Instruments).
6 6
D
total volume of 900 μL. The solutions were transferred into five pressure
NMR tubes and sealed with Teflon screw caps. The conversions were
1
monitored by H NMR spectroscopy.
tBu,Me
tBu,Me
Synthesis of the Complex Tp
Cu(NCMe) (1). TlTp
Alternative Determination of k Using TEMPO. Complex 1
1
-
2
-2
(0.81 mmol) and cuprous chloride, CuCl (0.82 mmol), were stirred in
(2,76 ꢀ 10 mmol) and TEMPO (3.6 ꢀ 10 mmol) were dissolved in
CH CN (20 mL) for 1 h. The mixture was filtered, and the colorless
5 mL of deoxygenated toluene in a Schlenk flask. After stirring for 30 min
3
-
4
solution was concentrated under vacuum and cooled at -20 ꢀC to give a
at 30 ꢀC, 1 mL of deoxygenated CCl
4
(6 ꢀ 10 mmol) and biphenyl
tBu,Me
-4
white crystalline solid of composition Tp
Cu(NCMe) in 80% yield.
(6 ꢀ 10 mmol) toluene stock solution were added to the Schlenk flask
via a degassed syringe. A sample was taken immediately for the reference,
and other samples were taken at timed intervals to measure kinetics. The
samples were passed through alumina to remove the catalyst. The
composition of the samples was analyzed by gas chromatography with
a Varian GC-3900 chromatograph with a flame ionization detector.
Computational Details. Gas phase optimizations were carried
1
H NMR (400 MHz, C D ) δ = 0.86 (br s, 3 H), 1.59 (s, 27 H), 2.35
s, 9 H), 5.85 ppm (s, 3H); C NMR (100 MHz, C
6
6
13
(
6
D
6
) δ = 2.5, 12.4,
3
2
1.2, 103.2, 144.5, 163.5 ppm. IR (Nujol) ν(CN) = 2206, ν(B-H) =
369 cm . For crystallographic data of 1 see Supporting Information.
-1
Reaction of 1 with CCl . A toluene solution (30 mL) of complex
4
1
4
(0.13 mmol) and CCl (200 mmol) was stirred for 4 h. A red brown
1
7
solution was obtained. The volatiles were removed under vacuum, and
the residue was extracted with diethyl ether. The resulting solution was
concentrated under reduced pressure. Crystallization at -30 ꢀC afforded
a red crystalline solid, for which the IR, NMR, and analytical data were
out using the ONIOM(QM:MM) approach as implemented in the
Gaussian03 package. In the ONIOM approach, the full system is divided
into two different regions; the electronically sophisticated environment, Cu
18
H
center, and Tp are treated with density functional theory (DFT), while
tBu,Me
9
t
tBu,Me
identical to those reported for Tp
CuCl.
the - Bu and -Me substituents of Tp
are treated with the MM
19
Kinetic Study of the Dissociation of Acetonitrile from 1. A
approach. For the QM part, the PBE1PBE functional was applied. The
-
2
total of 2.01 ꢀ 10 mmol of 1 were dissolved in C
6
D
6
(0.6 mL),
SDD basis set and associated effective core potential with a single f
20
transferred into a pressure NMR tube, and sealed with a Teflon screw
cap. The relative concentrations of 1 and 2 were monitored by H NMR
at 298 K with time using the resonances corresponding to the Bu.
polarization functionwasused for Cu; theTZVP basis set wasconsidered
1
21
22
for all atoms bonded to Cu, and TZV was used for other atoms. UFF
t
was employed in the MM region. All geometry optimizations were full with
no restrictions, and vibrational frequency calculations were also performed
to establish that the stationary points were minima or transition states. All
energies presented are potential energies.
Equilibrium Constant Measurement. To solution of 15 mg
-
2
(
1
1.50 ꢀ 10 mmol) of 2 in C
6 6 3
D (0.6 mL), 1 equiv of CH CN and
equiv of biphenyl, as internal standard, were added. The mixture was
transferred into a pressure NMR tube and sealed with a Teflon screw
X-ray Crystal Structure Analysis of 1. A single crystal of 1 of
suitable size was mounted on a glass fiber using perfluoropolyether oil
(FOMBLIN 140/13, Aldrich) in the cold N stream of a low-tempera-
2
cap. The relative concentrations of 1 and 2 were measured by integra-
t
tions of the resonances corresponding to the Bu protons of each
compound, after 100 h, when the equilibrium was reached.
ture device attachment. Full crystallographic data and structure refine-
ment are given in the Supporting Information. Intensity data were
performed on a Bruker-AXS X8 Kappa diffractometer equipped with
an Apex-II CCD area detector, using a graphite monochromator Mo
Kinetics of ATRA of CCl to Styrene at Different Tempera-
tures. A mixture of styrene (0.84 mmol), complex 1 (2.8 ꢀ 10 mmol
4
-
3
from a stock solution), and CCl (9.3 mmol) was dissolved in the required
4
amount of C D
6 6
to a total volume of 1.86 mL; [catalyst] = 1.5 mM.
KR
1
(λ = 0.71073 Å) and a Bruker Cryo-Flex low-temperature device.
2
465
dx.doi.org/10.1021/ic102279w |Inorg. Chem. 2011, 50, 2458–2467

Inorganic Chemistry
ARTICLE
The data collection strategy used in all instances was phi and omega
scans with narrow frames. Instrument and crystal stability were evaluated
from the measurement of equivalent reflections at different measuring
Wlodarczak, L.; Demonceau, A.; Noels, A. F. Eur. J. Org. Chem. 2001,
2689–2695. (e) Simal, F.; Wlodarczak, L.; Demonceau, A.; Noels, A. F.
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Demonceau, A.; Noels, A. F.; Vi ~n as, C.; Teixidor, F. Tetrahedron Lett.
2
3
times, and no decay was observed. The data were reduced (SAINT)
and corrected for Lorentz and polarization effects, and a semiempirical
2
003, 44, 8421–8425. (g) Tutusaus, O.; Vi ~n as, C.; N ꢀu n~ ez, R.; Teixidor,
24
F.; Demonceau, A.; Delfosse, S.; Noels, A. F.; Mata, I.; Molins, E. J. Am.
Chem. Soc. 2003, 125, 11830–11831. (h) de Clercq, B.; Verpoort, F.
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O.; Scopelliti, R.; Severin, K. Organometallics 2008, 27, 4464–4474. (j)
Lundgren, R. J.; Rankin, M. A.; McDonald, R.; Stradiotto, M. Organo-
metallics 2008, 27, 254–258. (k) Dutta, B.; Solari, E.; Scopelliti, R.;
Severin, K. Organometallics 2008, 27, 423–429. (l) Borguet, Y.; Richel,
A.; Delfosse, S.; Leclerc, A.; Delaude, L.; Demonceau, A. Tetrahedron
Lett. 2007, 48, 6334–6338. (m) Motoyama, Y.; Hanada, S.; Shimamoto,
K.; Nagashima, H. Tetrahedron 2006, 62, 2779–2788. (n) Motoyama, Y.;
Hanada, S.; Niibayashi, S.; Shimamoto, K.; Takaoka, N.; Nagashima, H.
Tetrahedron 2005, 61, 10216–10226. (o) Quebatte, L.; Haas, M.; Solari,
E.; Scopelliti, R.; Nguyen, Q. T.; Severin, K. Angew. Chem., Int. Ed. 2005,
44, 1084–1088. (p) Quebatte, L.; Scopelliti, R.; Severin, K. Eur. J. Inorg.
Chem. 2005, 3353–3358. (q) Fern ꢀa ndez-Z ꢀu mel, M. A.; Thommes, K.;
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absorption correction was applied (SADABS). The structure was
2
5
2
solved by direct methods (SIR-2002) and refined against all F data
26
by full-matrix least-squares techniques (SHELXTL-6.14) minimizing
w[Fo - F ] . All the non-hydrogen atoms were refined with aniso-
2
2 2
c
tropic displacement parameters. The hydrogen atoms were introduced
into the geometrically calculated positions and refined riding on the
corresponding parent atoms.
Crystal Data for 1. C28
crystal of dimensions 0.50 ꢀ 0.39 ꢀ 0.32 mm , monoclinic, space group
P2 /c, a = 25.0527(14) Å, b = 15.5161(9) Å, c = 17.9635(9) Å, R = 90ꢀ,
β = 110.9970(10)ꢀ, γ = 90ꢀ, V = 6519.1(6) Å , T = 173(2) K, Z = 8, D =
8 w
H46BCuN , M = 569.08, colorless prism
3
1
3
3
-1
1
.160 Mg/m , F = 0.699 mm , F(000) = 2432; 134670 reflections
measured, of which 19771 were unique (Rint = 0.0618), 769 refined
parameters, final R = 0.0598 for reflections with I > 2σ(I), wR = 0.1521
1
2
(
all data), GOF = 1.084. Final largest diffraction peak and hole: 1.590
-
3
2
009, 15, 11601–11607.
4) (a) Sasson, Y.; Rempel, G. L. Synthesis 1975, 448–450. (b)
Grove, D. M.; Van Koten, G.; Verschuuren, A. H. M. J. Mol. Catal. 1988,
5, 169–174. (c) van de Kuil, L. A.; Grove, D. M.; Gossage, R. A.;
and -1.029 e Å . See Supporting Information for supplementary
crystallographic data.
3
(
4
’
ASSOCIATED CONTENT
Zwikker, J. W.; Jenneskens, L. W.; Drenth, W.; van Koten, G. Organo-
metallics 1997, 16, 4985–4994.
S
Supporting Information. NMR data for the products,
b
(5) (a) Pintauer, T. Eur. J. Inorg. Chem. 2010, 2449–2460. (b)
NMR spectra of reaction mixtures, and kinetic plots (PDF) and
CIF file. This material is available free of charge via the Internet at
http://pubs.acs.org. CCDC 775708 contains the supplementary
crystallographic data for the compounds described in this paper.
These data can be obtained free of charge via www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax þ44
Eckenhoff, W. T.; Garrity, S. T.; Pintauer, T. Eur. J. Inorg. Chem. 2008,
5
9
63–571. (c) Balili, M. N. C.; Pintauer, T. Inorg. Chem. 2009, 48, 9018–
026. (d) Eckenhoff, W. T.; Pintauer, T. Inorg. Chem. 2007, 46, 5844–
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Biernesser, A. B.; Noonan, S. J.; Taylor, M. J. W. Chem.—Eur. J. 2009, 15,
38–41.
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Diaz-Requejo, M. M.; Trofimenko, S.; Belderrain, T. R.; P ꢀe rez, P. J.
Inorg. Chem. 2007, 46, 7725–7730.
1
223-336-033; or deposit@ccdc.cam.ac.uk).
AUTHOR INFORMATION
Corresponding Author
’
(
7) Mu ~n oz-Molina, J. M.; Belderrain, T. R.; P ꢀe rez, P. J. Adv. Synth.
Catal. 2008, 350, 2365–2372.
8) (a) Fujisawa, K.; Ono, T.; Ishikawa, Y.; Amir, N.; Miyashita, Y.;
(
*
(
E-mail: trodri@dqcm.uhu.es (T.R.B.) and perez@dqcm.uhu.es
P.J.P.).
Okamoto, K.-i.; Lehnert, N. Inorg. Chem. 2006, 45, 1698–1713. (b) Yap,
G. P. A.; Jove, F.; Urbano, J.; Alvarez, E.; Trofimenko, S.; Díaz-Requejo,
M. M.; P ꢀe rez, P. J. Inorg. Chem. 2007, 46, 780–787. (c) Schneider, J. L.;
Carrier, S. M.; Ruggiero, C. E.; Young, V. G., Jr.; Tolman, W. B. J. Am.
Chem. Soc. 1998, 120, 11408–11418.
’
ACKNOWLEDGMENT
We thank the MICINN (Grants CTQ2008-00042BQU,
CTQ2008-06866-CO2-02/BQU, and Consolider Ingenio 2010
CSD2006-0003) and the Junta de Andalucía (Proyecto P07-
FQM-02794) for financial support.
(9) Yoon, K.; Parkin, G. Polyhedron 1995, 14, 811–821.
(
10) Martín, C.; Mu ~n oz-Molina, J. M.; Locati, A.; Alvarez, E.;
Maseras, F.; Belderrain, T. R.; P ꢀe rez, P. J. Organometallics 2010, 29,
481–3489.
11) Bartolomei, N.; Isse, A. A.; Di Marco, V. B.; Gennaro, A.;
3
(
’
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dx.doi.org/10.1021/ic102279w |Inorg. Chem. 2011, 50, 2458–2467